1. Field of the Invention
The present invention relates to a diagnostic apparatus for a valve timing control system, particularly for a valve timing control system in which a rotational phase between a crankshaft and a camshaft of an internal combustion engine is designed so as to change.
2. Discussion about Prior Arts
In recent years, an engine incorporating a valve timing control system in which a rotational phase between a crankshaft and a camshaft of the engine is adjustable, has been put into practical use. Generally, the valve timing control system has a variable valve timing mechanism for continuously varying at least either of an intake valve timing and an exhaust valve timing.
Since the valve timing is one of very important engine parameters, the valve timing control system needs a diagnostic apparatus in case of failures. For example, Japanese Patent Application Laid-open No. Toku-Kai-2001-20798 discloses a technique in which frequency of misfires is monitored for every operating area and in case where the frequency of misfires is high only at a low speed and low load operating area, it is judged that the high speed cam on the exhaust side is stuck, and in case where the frequency of misfires is high at low speed and low load operating areas and at intermediate speed and intermediate load operating areas, it is judged that the high speed cam on the intake side is stuck.
However, according to the technology wherein the frequency of misfires is calculated for every operating area of the engine as described in Toku-Kai-2001-20798, a burden of the calculation of the frequency on the computer increases and has such adverse effects as delays in judgments, detection errors and the like. Further, since the misfire judgments are made only at low speed and low load operating areas and at intermediate speed and intermediate load, there is a disadvantage that the range of diagnoses is restricted.